Date: Mon, 17 Jul 2017 05:39:38 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Tony Lindgren <tony@atomide.com>
Cc: Alex Shi <alex.shi@linaro.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Linux OMAP Mailing List <linux-omap@vger.kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <Will.Deacon@arm.com>
Subject: Re: Regression with suspicious RCU usage splats with cpu_pm change
Reply-To: paulmck@linux.vnet.ibm.com
References: <20170713070749.GE16509@atomide.com>
 <c00272dc-cf84-81a2-b2e2-d52cadf7bcc6@linaro.org>
 <CAJZ5v0j15uWyCRAPep4KwqSiC4UD4ZppAG++SUvN3Ok1wzDwNQ@mail.gmail.com>
 <20170713124318.GK2393@linux.vnet.ibm.com>
 <c5c6623b-ea1d-1bbf-ea80-5d93e7e81c6f@linaro.org>
 <20170717060806.GC10026@atomide.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170717060806.GC10026@atomide.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Message-Id: <20170717123938.GP2393@linux.vnet.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1236
Lines: 31

On Sun, Jul 16, 2017 at 11:08:07PM -0700, Tony Lindgren wrote:
> * Alex Shi <alex.shi@linaro.org> [170716 16:25]:
> > I reused the rcu_irq_enter_irqson() from RCU_NONIDLE to avoid this issue. 
> > It works fine.
> > 
> > Tony, Could you like to give a tested-by if this patch works for you.
> 
> Yeah that keeps things booting for me with no splats so:
> 
> Tested-by: Tony Lindgren <tony@atomide.com>
> 
> In general, it seems we're missing the knowledge in Linux kernel
> of when the entire system is idle. Right now it seems that only
> cpuidle_coupled knows that?
> 
> We could probably simplify things by adding some PM state for
> entire system idle. Then cpuidle code and timer code could use
> that to test when it's safe to do whatever the SoC needs to do
> to enter deeper power states.
> 
> If we already have something like that, please do let me know :)

Well, we used to have CONFIG_NO_HZ_FULL_SYSIDLE, which detected
full-system idle lazily so as to avoid scalability bottlenecks.
https://lwn.net/Articles/558284/

No one was using it, so I removed it last merge window.  The
patch that removed it is at sysidle.2017.05.11a, which can
probably still be reverted cleanly.  Or just use v4.11 or earlier.

						Thanx, Paul